The present invention relates generally to medical procedures in which a device is inserted into a body, and more particularly to tracking of such device with the use of magnetic resonance signals.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a rotating magnetic field (excitation field B1) which is in the x-y plane and which is rotating near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. When such a rotating magnetic field is applied for a finite duration, the resulting pulse is termed an “excitation rf pulse”. The net transverse magnetic moment created by the excitation RF pulse also rotates at the Larmor frequency and continues after the excitation RF pulse is terminated. This transverse magnetic moment creates a detectable MR signal that may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Using an alternate sequence of measurement cycles it is possible to determine the location of a device placed within a body. In one such sequence one or more small MR receive coils acquire MR signals in the presence of a magnetic field gradient (Gx, Gy or Gz). Fourier analysis is applied to the detected MR signal to determine the frequency of the signal. Since the signal is acquired in the presence of a selected magnetic field gradient and since the signal is detected with a small receive coil having limited spatial sensitivity, the frequency of the signal provides a measure of the coil's (and subsequently the device's) location. Using this or one of many other well-known active MR tracking techniques it is possible to rapidly determine the three-dimensional coordinates of one or more coils.
While tracking an active device in a fluid having a long longitudinal relaxation time, T1, such as blood, the signal is decreased because of the reduction of longitudinal polarization associated with the long T1 and the rapid application of RF excitation pulses. Under some conditions, such as in an environment of high flow rate, this leads to compromised tracking capability. For example, in the atrial chambers of the heart, which are of large interest for electrophysiology procedures, tracking capability can be less than ideal.
Also, spins that are not near the active device can generate MR signals that couple to volume RF coils in the scanner, such as the radio-frequency body coil. These signals can then couple back to the tracking coils. The result is that spins that are not near the device are detected. Under conditions of low signal-to-noise, this confounding signal interferes with the localized signal desired from the device and tends to compromise the device tracking.
It would therefore be desirable to have a system and method capable of reducing interference to tracking signals when tracking an active device in an MR scan. It would also be desirable to have a system and method capable of robust tracking in the presence of moving blood having a long longitudinal relaxation time, T1.